The present invention relates to a new and improved sheet material handling apparatus in which the speed of movement of a plurality of belts is coordinated to prevent unintended deformation of the sheet material.
A sheet material folding apparatus is disclosed in a copending U.S. Pat. application filed by Richard E. Breton and David B. Staley and entitled "Folder Apparatus". This application discloses a folder apparatus which is operable at a relatively high speed to form a plurality of folds in sheet material. The folds are formed by a series of folder assemblies through which the sheet material continuously moves at a relatively high speed. The folded sheet material is stacked on edge by a stacker assembly.
One of the folder assemblies includes a tapered array of belts which move the sheet material to be folded into the folder assembly. As the sheet material is moved by the tapered array belts,, it is engaged by a pair of creaser belts. The creaser belts grip opposites sides of the sheet material and move the sheet material out of the folder assembly.
During operation of this folder assembly, the speed of movement of the sheet material by the tapered array of belts and by the creaser belts must be accurately coordinated so that the sheet material can be gripped by the creaser belts while it is being moved by the tapered array of belts. Once the sheet material has been gripped by the creaser belts, the speed of movement of the upper and lower creaser belts must be accurately coordinated in order to prevent unintended deformation of the sheet material. Thus, if one of the creaser belts is moving faster than the other creaser belt, one side of the sheet material will be pulled forwardly relative to the other side of the sheet material.
Difficulties are encountered in coordinating the speed of movement of the creaser belts. This is because, the effective pitch diameters of the pulleys or sheaves which support and drive the creaser belts varies during operation of the folder. Variations in the effective pitch diameters of the creaser belt pulleys are due to several different factors. Included among these factors are: (1) the V-shaped configuration of cooperating surfaces on the pulleys and the creaser belts and (2) variations in the cross-sectional size of the creaser belts along the length of the creaser belt.
The creaser belt pulleys are of the well known V-type. A slight change in the spacing between the side surfaces of a creaser belt which engages a pulley results in a larger change in the pitch diameter of the pulley. Thus, if the distance between the side surfaces of a creaser belt decreases slightly, the creaser belt will move a larger distance inwardly toward the center of rotation of the pulley which it engages. Similarly, if the distance between the side surfaces of the creaser belt increases, the creaser belt will move a larger distance outwardly away from the center of rotation of the pulley. Therefore, the cooperation between the creaser belts and pulleys effectively magnifies any change in the cross section of the creaser belts.
The creaser belts may be commercially fabricated by an extrusion process. During extrusion of a creaser belt, it is extremely difficult to hold very close tolerances on the cross sectional size of the creaser belt. Therefore, the cross section of the creaser belt, as originally formed, varies along the length of the belt. During use, the creaser belt will tend to stretch due to tension on the belt and centrifugal effects. Of course, stretching of the creaser belt will change the cross sectional size of the belt.
The foregoing and other factors make it very difficult to maintain the effective pitch diameter of the pulleys around which the creaser belts extend constant during movement of the belts. Of course, if the effective diameter of the pulleys which engage the creaser belts vary during movement of the belts, the speed of movement of the creaser belts will vary. When the speeds of creaser belts vary, one of the belts will move further than the other belt. Thus, in a test, one creaser belt of a pair of creaser belts moved 1.25 inches further than the other belt in four feet of belt travel. Of course, if creaser belts engaging opposite sides of a signature travel through different distances, the signature will be deformed by the creaser belts.